Surface ornamentation for a lunch bag



FIG. 1 is a front perspective view of a surface ornamentation for a lunch bag showing my new design;

FIG. 2 is a front elevation view thereof;

FIG. 3 is a side elevation view thereof, the opposite side being a mirror image; and,

FIG. 4 is a top plan view thereof.

The broken lines showing the lunch bag are for illustrative purposes only and form no part of the claimed design. 

The ornamental design for a surface ornamentation for a lunch bag, as shown and described. 